The proposed research is a neuroethological study of social communication among electroreceptive fishes of South America (the Gymnotiformes) and Africa (the Mormyridae). Electrical fish are seen as a model system for a mechanistic study of communication in a vertebrate. The system is favorable because it includes many of the rich complexities that we associate with a vertebrate social communication system while employing signals that are relatively simple to describe and to reproduce, and a sensory modality whose characteristics can be carefully documented. This research will examine the mechanisms of electric signal production and evolution, and will explore the neuronal basis for electric signal recognition among pulse-type electric fish. Many of the African and South American electric fish produce pulsatile electric organ discharges (EODs) which serve in species- and sex-recognition. In addition, they discharge these pulses according to patterned sequences of pulse intervals (SPIs) which serve in display functions including courtship, alarm, appeasement and threat. The first goal of this study is to describe the variation in the structure of the communication signals among the pulse gymnotiform fishes of the genus, Hypopomus from South America. Members of this genus differ in the waveform, duration, spectral characteristics, and amplitude of their species-typical EODs; males and females also differ with regard to these parameters. Animals will be collected in their natural habitat order to catalogue the variation in EODs. The amplitudes of discharges will be measured in order to determine the distance over which signals are effective in their own natural habitats, and under conditions of artificially-altered water conductivity. These comparative studies will provide a quantitative database for understanding signal variation within populations of animals, and a basis for understanding how signals evolve. Behavioral observations will be conducted to record the electrical display repertoire of Hypopomus both under field conditions, and laboratory conditions. Playback experiments will be conducted to study the ability of Hypopomus to recognize electric discharges and to discriminate between them. The experiments will explore species and sex recognition in Hypopomus. Electrophysiological studies will be run in parallel to describe the sensory coding of natural EOD stimuli by peripheral electroreceptors in Hypopomus. The electric stimuli will be presented to the animals in a naturalistic way. Electrophysiological studies will be conducted in mormyrids at sensory processing of electric discharges at the level of the midbrain.